Cowichan Head Bluffs and Aquifer EarthCache
Hidden : 10/17/2013
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: other (other)

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Geocache Description:


To log this EarthCache, you need to send me a message with the answers to the questions at the end of the description. Failure to do so may result in your log being deleted. I also ask that you don't "save" your find for a different day (for example to extend a streak, fill a hole in your calendar, etc.). Please log your find for the day you visited the location.

Parking is available at the end of Island View Road between 6 am and 11 pm. Please keep in mind that you'll need to walk over a kilometre along the beach, so check the tides to make sure you'll have time to get there and back. Based on Saanichton Bay tides (tide table), you'll need the water level to be about 6-7 feet or less to get to the posted coordinates without getting your feet wet. South of Island View Road, everything above the high-tide mark is private property, so make sure you stay on the beach.

Cowichan Head Bluffs and Aquifer


The bluffs above Island View Beach are a great place to experience the story of the last 100,000 years of the geological history of the Saanich Peninsula. Just north of Cowichan Head, where the ground has eroded away into a nearly vertical cliff, the many layers of sediment laid down by various geological processes can easily be seen. You can also see traces of a local aquifer, where water stored in one of the sedimentary layers is seeping out of the exposed face of the bluff.

Sedimentary Layers
The bluffs at Cowichan Head are made up of many layers of sediment, each consisting of different types of material and laid down by different geological processes. Almost all of the layers are somehow related to the massive sheet of ice, called the Cordilleran ice sheet, that covered all of British Columbia and the US Pacific Northwest during the last ice age. The Cordilleran ice sheet started to form about 2.6 million years ago, and most of it had melted away by about 10,000 years ago. The glaciers that still exist in the central part of Vancouver Island and the coastal mountains of the mainland are all that remains of the Cordilleran ice sheet. When it covered this region, the sheer weight of the ice sheet (it was between 1 and 1.5 km thick!) pushed down on the earth so much that everything you see in the bluffs above you was at or below sea level at one time. In the last 100,000 years, the ice sheet was responsible in some way for creating most of the 7 major sedimentary layers that now cover the eastern part of the Saanich Peninsula.


Sedimentary Layers at Waypoint A

Muir Point Formation
The Muir Point Formation is the oldest layer visible at Cowichan Head. It was formed during the Sangamon interglacial period, a time when the Cordilleran ice sheet receded and no longer covered this region. From about 125,000 to 80,000 years ago, sediment created by earlier glacial activity (called till), silt, sand and gravel was washed down into coastal floodplains by rivers, streams and various other forms of erosion to create this layer. At Waypoint A, the Muir Point Formation is the light- to dark-grey layer of clay that starts at beach level and rises up about 10 metres. Due to the way the layers angle down to the south, the Formation is below beach level at the posted coordinates.

Dashwood Drift
The layer above the Muir Point Formation is known as the Dashwood Drift. It was formed in the early part of the Wisconsin glaciation period, from about 80,000 to 65,000 years ago. The Wisconsin glaciation was a period when the Cordilleran ice sheet expanded and again covered this region. As ice gradually melted on the ice sheet, the water carried silt, silty sand and till into a shallow estuary, leading to one of the more notable features of the bluffs: fossils. The sediment buried the remains of marine organisms that lived in the estuary during that time period, and those remains can now be seen at the exposed face of the Dashwood Drift. Interestingly, the species found here as fossils are still alive today over 1000 kilometres north in Alaska, where the climate is similar to what the climate was like in southern BC over 65,000 years ago. At Waypoint A, the Dashwood Drift is the 3 metre layer of grey clay above the Muir Point Formation and just below the lowest layers of brown clay. At the posted coordinates, the Dashwood Drift is the 1.5 metre layer right at beach level.

Cowichan Head Formation
The Cowichan Head Formation, which is named after this very spot, was created during the middle portion of the Wisconsin glaciation from 65,000 to 29,000 years ago. The Cordilleran ice sheet still covered much of the western part of the continent, but had receded in this area during a period called the Olympia interglaciation (a period between glaciation periods). During this time, the land was rising because the weight of the ice sheet was no longer pushing down on it. This change of sea level caused the Cowichan Head Formation to have two different types of sediment. The lowest consists of marine sediment: sand and gravel that were washed down by water from the melting ice sheet and deposited in shallow salt water. As the land rose, the sediment being deposited changed and became silt from the newly-exposed sea floor, which collected in coastal floodplains and the estuaries of rivers and streams. Much of the sediment of the Cowichan Head Formation has oxidized, giving it a brown colour. This 3-4 metre layer of brown clay can be seen just above the grey clay of the Dashwood Drift at both Waypoint A and the posted coordinates.

Quadra Formation
The Quadra Formation, also known as the Quadra Sand, formed between 29,000 and 23,000 years ago during the later portion of the Wisconsin glaciation period. The Cordilleran ice sheet advanced and covered this area for the last time during a period termed the Fraser glaciation. As the ice sheet advanced, sand and gravel were washed down in front of it and deposited in coastal floodplains, estuaries and deltas in well-sorted layers. Due to the well-sorted nature of the sediment and the fact that this layer is as deep as 30 metres in some places, this sand and gravel is commercially extracted at several locations including the Butler Brothers gravel pit on Keating Cross Road. Another interesting feature is that mammoths were common in this region during the time that this layer was forming, and remains of mammoths have been found as the Quadra Formation erodes and the remains fall down to the beach. At both the posted coordinates and Waypoint A, the Quadra Formation consists of a series of layers of sand and gravel and is the highest layer visible.

Vashon Till
In a few places along the southernmost portions of the bluffs, a small amount of the layer above the Quadra Formation can be seen. This layer is called the Vashon Till and consists of sandy and gravely clay till deposited while the Cordilleran ice sheet had expanded to its greatest during the Fraser glaciation between 23,000 and 14,000 years ago.

Other Layers
As the ice sheet retreated at the end of the Fraser glaciation (also marking the end of the Wisconsin glaciation) between 14,000 and 11,700 years ago, sand, gravel and silt were deposited in a layer known as the Capilano Sediments. After the ice sheet had completely receded, in the current time period known as the Holocene, the deposited sediment is known as the Salish Sediments. This includes river sediment, organic lake sediment, and wetland and estuary sediment. Both the Capilano and Salish Sediments cover parts of the Saanich Peninsula, but can't be seen at the Cowichan Head bluffs.


Sedimentary Layers at the Posted Coordinates

Aquifer
An aquifer is a layer of unconsolidated material (gravel, sand, silt or clay) or rock that is permeable (allows water to pass through it) and contains water. Water can be extracted from an aquifer by digging a well down into the aquifer and allowing water to seep from the surrounding layers into the hole. Depending on the quality of the water, it can be used as drinking water or simply for agricultural use.

Cowichan Head Aquifer
The Cowichan Head Aquifer is a 3.4 square kilometre aquifer located beneath the nearby ridge, extending from Cowichan Head in the south, almost to Mount Newton Cross Road in the north, and from the coast to 0.8-1.5 kilometres inland. The water is primarily found in the sandy Quadra Formation layer, but can also be found lower in the Cowichan Head Formation. On top of the Quadra Formation, in the Vashon Till layer, is a fine-grained brown or grey clay called Victoria Clay that's found above most of the Cowichan Head Aquifer. The Victoria Clay forms a confining layer that slows the penetration of rainwater through to the aquifer, but also helps stop the aquifer from being contaminated by human activity at the surface, such as by fertilizer from farming. Due to this layer covering some but not all of the aquifer, it's classified as a partially confined aquifer.

While the sand and gravel that makes up the Cowichan Head Aquifer is usually associated with good aquifers, the confining layer of Victoria Clay slows or blocks rainwater from getting to the aquifer, so this aquifer is considered to have low production. Some of the water seeps out of the exposed edge of the aquifer at the Cowichan Head bluffs, which you can see at both the posted coordinates and Waypoint A. If it hasn't rained recently and the rest of the bluff is dry, you should be able to see the water seeping out of the sandy Quadra Formation and the rusty-brown Cowichan Head Formation and running down over the thick, grey clay of the Dashwood Drift. The water seeps out at this level because the dense clay of the Dashwood Drift stops the water from transferring any deeper into the ground.

Questions
To log this Earthcache, please send me a message with the answers to the following questions (don't include them in your log):

1. At the posted coordinates, take a close look and describe the grey clay of the Dashwood Drift (the lowest 1.5 metres of the bluff). What does it look like? Can you see anything embedded in it?

2. At either the posted coordinates, Waypoint A (N48 33.870 W123 22.050), or the alternate GZ (N48 33.769 W123 22.059), describe how much water is seeping out of the aquifer. For example, is it a steady flow, a light trickle, or just a faint trace of water?

3. At Waypoint A, using the depth of the visible layers as a guide, estimate the height of the bluffs.

4. What's the term for sediment desposited by glacial activity?

You don't need to wait for a response from me before logging your find. If there are any problems, I'll contact you.

Bibliography
  • Sylvia Kenny, "Aquifers of the Capital Regional District", December 2004
  • Grant Keddie, "The Mammoth Story"
  • David Huntley, Katrina Bennett, Peter Bobrowsky, John Clague, "Surficial Geology and Geomorphology of Central Saanich Peninsula, Southeastern Vancouver Island"
  • Melvyn E. Best, Bemex Consulting International, "Groundwater Aquifers"
  • Wikipedia

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